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Predicting Martian Dune Characteristics Using Global and Mesoscale MarsWRF Output . Claire Newman working with Nick Lancaster , Dave Rubin and Mark Richardson. Acknowledgements: funding from NASA’s MFR program, and use of NASA’s HEC facility right here at Ames. Overview of talk.
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Predicting Martian Dune Characteristics Using Global and MesoscaleMarsWRFOutput Claire Newman working with Nick Lancaster, Dave Rubin and Mark Richardson Acknowledgements: funding from NASA’s MFR program, and use of NASA’s HEC facility right here at Ames.
Overview of talk • Motivation (why dunes?) • Dune features we can compare with • Some dune theory • Modeling approach • Preliminary results: Global • Preliminary results: Gale Crater
Motivation: how do dunes provide insight into recent climate change on Mars? • Higher obliquity should produce stronger circulations & surface stresses, hence might expect more saltation and dune formation • Features (i.e., dune orientations, migration directions, etc.) in disagreement with predictions for current wind regime may indicate inactive dunes formed in past orbital epochs Also – • In absence of near-surface meteorological monitoring, predicting characteristics of active dunes can help us confirm that we • have the current wind regime right • understand dune formation processes
Dune features we can compare with • Locations Bourke and Goudie, 2009
Dune features we can compare with • Locations • Bedform (crest) orientations Images: NASA/JPL/University of Arizona
Dune features we can compare with • Locations • Bedform (crest) orientations Images: NASA/JPL/University of Arizona
Dune features we can compare with • Locations • Bedform (crest) orientations • Inferred migration directions (crater dunes) Hayward et al., 2009
Dune features we can compare with • Locations • Use a numerical model to predict saltation over a Mars year, assuming a range of saltation thresholds • Bedform (crest) orientations • Apply ‘Gross Bedform-Normal Transport’ theory • Inferred migration directions (crater dunes) • Assume correlated with resultant (net) transport direction
2 key issues • Dunes form in the long-term wind field • Dune orientations are not determined by net transport
Gross Bedform-Normal Transport A simple example to illustrate a point Dune crest Rubin and Hunter, 1987
Gross Bedform-Normal Transport A simple example to illustrate a point First wind direction Dune crest
Gross Bedform-Normal Transport A simple example to illustrate a point First wind direction Dune crest Second wind direction
Gross Bedform-Normal Transport A simple example to illustrate a point First wind direction Net transport = 0 Dune crest Second wind direction
Gross Bedform-Normal Transport A simple example to illustrate a point First wind direction Net transport = 0 But both wind directions shown cause the bedform to build Dune crest Second wind direction
Gross Bedform-Normal Transport A simple example to illustrate a point First wind direction Net transport = 0 But both wind directions shown cause the bedform to build => rather than net transport, we are interested in gross transport perpendicular to the bedform, regardless of the ‘sense’ of the wind (i.e., N-S versus S-N) Dune crest Second wind direction
Bedform orientation – the theory A A = NET TRANSPORT OF SAND
Bedform orientation – the theory B C B+ C = GROSS BEDFORM-NORMAL SAND TRANSPORT
Bedform orientation – the theory • Basic concept: dunes form due to sand transport in both directions across bedform • Bedforms align such that total transport across dune crest is maximum in a given wind field • where total transport = Gross Bedform-Normal Transport [Rubin and Hunter, 1987] B C B+ C = GROSS BEDFORM-NORMAL SAND TRANSPORT
Bedform orientation – the approach • Run numerical model to predict near-surface winds at all times for a long time period (at least 1yr) to capture the long-term dune-forming wind field • Choose a saltation threshold and calculate sand fluxes in all directions [0, 1, …359° from N] • Consider all possible bedform orientations [0, 1, …179° from N] • Sum the gross sand flux perpendicular to each orientation over the entire time period • Find the orientation for which the total gross flux is maximum • NB: secondary maxima indicate secondary bedformorientations
The numerical model: MarsWRF • Mars version of planetWRF (available at www.planetwrf.com) • Developed from Weather Research and Forecasting [WRF] model widely used for terrestrial meteorology • Multi-scale 3D model capable of: • Large Eddy Simulations • Standalone mesoscale • Global • Global with nesting • Using global and global with nesting for these studies
Version of MarsWRF used here includes: • Seasonal and diurnal cycle of solar heating, using correlated-kradiativetransfer scheme (provides good fit to results produced using line-by-line code) • CO2 cycle (condensation and sublimation) • Vertical mixing of heat, dust and momentum according to atmospheric stability • Sub-surface diffusion of heat • Prescribed seasonally-varying atmospheric dust (to mimice.g. a dust storm year or a year with no major storms) or fully interactive dust (with parameterized dust injection) • Ability to place high-resolution nests over regions of interest
Present day global dune results comparing with Mars Global Digital Dune Database (MGD3, e.g.Hayward et al., 2009):1. Dune centroid azimuth – compare with GCM-predicted resultant transport direction
Present day global dune results comparing with Mars Global Digital Dune Database (MGD3, e.g.Hayward et al., 2009):1. Dune centroid azimuth – compare with GCM-predicted resultant transport direction2. Slipface orientation – compare with normal to GCM- predicted bedform orientation
Present day global dune results comparing with Mars Global Digital Dune Database (MGD3, e.g.Hayward et al., 2009):1. Dune centroid azimuth – compare with GCM-predicted resultant transport direction –agreement => within 45° of dune centroid azimuth direction2. Slipface orientation – compare with normal to GCM-predicted bedform orientation – agreement => within 45° of normal to slipface
Comparison of predicted and inferred migration direction for present day using saltation threshold=0 Green => agreement between predicted and inferred migration direction Blue => no agreement Red => no comparison possible
Comparison of predicted and inferred migration direction for present day using saltation threshold=0.007N/m2 Green => agreement between predicted and inferred migration direction Blue => no agreement Red => no comparison possible
Comparison of predicted and inferred migration direction for present day using saltation threshold=0.021N/m2 Green => agreement between predicted and inferred migration direction Blue => no agreement Red => no comparison possible
Comparison of predicted and measured bedform orientation direction for present day using saltation threshold=0 Green => agreement between predicted and inferredbedform orientation Blue => no agreement Red => no comparison possible
Comparison of predicted and measured bedform orientation direction for present day using saltation threshold=0.007N/m2 Green => agreement between predicted and inferredbedform orientation Blue => no agreement Red => no comparison possible
Comparison of predicted and measured bedform orientation direction for present day using saltation threshold=0.021N/m2 Green => agreement between predicted and inferredbedform orientation Blue => no agreement Red => no comparison possible
Comparison of predicted and inferred migration direction for obliquity 35° using saltation threshold=0.007N/m2 Green => agreement between predicted and inferred migration direction Blue => no agreement Red => no comparison possible
Comparison of predicted and inferred migration direction for present day using saltation threshold=0.007N/m2 Green => agreement between predicted and inferred migration direction Blue => no agreement Red => no comparison possible
Nesting in MarsWRF One or more high-resolution ‘nests’, placed only over regions in which increased resolution is desired
Mother [global] domain (only a portion shown) 5°N Domain 2 (5 x resolution of domain 1) 0° Domain 3 (5 x resolution of domain 2; 25 x resolution of domain 1) 5°S 20°E 15°E 25°E 30°E 35°E
Mother [global] domain 2-way nesting => feedbacks between domains 5°N Domain 2 Domain 3 0° 1-way nesting => parent forces child only 5°S 20°E 15°E 25°E 30°E 35°E
Sample results for Gale at Ls ~ 0° using mesoscale nesting in global MarsWRF
Gale dune studies – early results (~4km resolution) Gale Crater: predicted (a) resultant transport direction [black arrows] (b) dune orientations [white lines] for saltation threshold = 0
Gale dune studies – early results (~4km resolution) Gale Crater: predicted (a) resultant transport direction [black arrows] (b) dune orientations [white lines] for saltation threshold = 0.007N/m2
Gale dune studies – early results (~4km resolution) Gale Crater: predicted (a) resultant transport direction [black arrows] (b) dune orientations [white lines] for saltation threshold = 0.021N/m2
Gale dune studies –present day Mars No saltation threshold Saltation threshold = 0.007N/m2
Gale dune studies –present day Mars No saltation threshold Saltation threshold = 0.007N/m2
Gale dune studies –present day Mars No saltation threshold Saltation threshold = 0.007N/m2 From Hobbs et al., 2010
Gale dune studies –present day Mars • Simply changing the assumed saltation threshold greatly improves the match to observed bedform orientations • Effect of large dust storms not yet examined, but likely will also impact winds hence orientations • Orbital changes and impact on circulation widen parameter space even further!